1. Field of the Invention
This invention relates to lenses and optical systems and, more particularly, to an aplanatic microlens and a method for making such a lens.
2. Brief Description of the Art
As the geometries of integrated circuits and other micro components have become smaller and smaller, a need for higher resolution optical instruments, and particularly lenses, has arisen. To meet this need, the optics industry has responded by designing and manufacturing lenses which are most effective for light of wavelengths in the deep ultraviolet.
The lenses so manufactured have been made using traditional lens making techniques and have, for the most part, led to unsatisfactory results in lenses with high numerical apertures for the following reasons:
(i) multi-element lenses (typically 10 or more) are much more sensitive to scratches, misalignment, and fabrication errors than lenses which are designed for visible light;
(ii) anti-reflection coatings ar difficult to fabricate for ultraviolet light wavelengths;
(iii) as a result of unavoidable differences in manufactured lenses, two lenses of the same design tend to show marked differences in imaging; and
(iv) the lenses, though faulty, are extremely expensive to custom manufacture.
More specifically, in the semiconductor industry there are two major dimensional metrology tasks: the line width or critical dimension measurement; and layer-to-layer overlay measurement. As there are a number of accuracy problems relating to optical critical dimension measurement, this measuring task has traditionally been performed by means of an electron microscope. However, electron microscopes cannot form images through transparent films and therefore optics must be used for many overlay measurements.
Furthermore, unless additional process steps are added to clear a hole in the transparent layer, electron beam imaging cannot be used to measure layer-to-layer overlay accuracy where the overlay target is buried beneath a transparent layer.
Unfortunately, today's optics can, as a rule of thumb, only be used to measure down to about 0.8 micrometers line widths; a restriction which makes today's optical instruments inadequate for line width and layer-to-layer overlay measurements required in state of the art semiconductor technologies. But if a lens existed which could operate in wavelengths in the deep ultraviolet (using a laser of around 200 nanometers wavelength), then it is conceivable that optical instruments can be used to measure line widths to below 0.4 micrometers. If a lens of this nature existed, it could also be used for defect inspection.
Finally, a new technology called phase shifting mask lithography has increased a need for good optical metrology in the semiconductor industry because the optical thickness of the small lines must be measured as well as their critical dimensions. So, high quality optical lenses which are highly reproducible are desired.
There is therefore a need, particularly for purposes of inspection and measurement in semiconductor fabrication operations, for a highly reproducible and simple lens which can operate in wavelengths in the deep ultraviolet.